1. Field of the Invention
The present invention relates to a process for thin film formation, particularly to a process for thin film formation which is useful for fine working technique for producing electronic apparatuses such as electronic devices and integrated circuits employing a semiconductor or a thin metal film, and fine mechanisms such as micromachining.
2. Related Background Art
Electronic devices and integrated circuits have attained high performance and high integration degree by extremely fine structure thereof. Today the minimum working dimension is 0.8 .mu.m in commercial 4M bit DRAM. The gate length of 0.07 .mu.m is reported for MOSFET in an experimental model. In a working dimension of 0.1 .mu.m or finer, electron wave interference and tunnel effect become remarkable. Electronic devices based on a new physical phenomenon are expected.
In order to achieve finer structure of electronic devices and integrated circuits, or to realize an electronic device based on a new physical phenomenon, technique for fine working needs to be established which enables stable working in a dimension of 0.1 .mu.m or finer. For the future, required working dimension will be 0.1 .mu.m or finer: for example, working of metal wiring of 100.ANG. wide and 100.ANG. thick at working accuracy of an .ANG. order.
Conventional fine working processes which employ an organic resist film are briefly described with reference to FIGS. 4A to 4D, and the problems in achieving a finer structure are considered.
In FIG. 4A, a thin film 402 is formed on a substrate 401. The substrate 401 may be an Si substrate, or an Si wafer with SiO.sub.2 formed thereon. The thin film 402 may be a film made of a metal such as Al (aluminum) or the like, or be an insulating film made of BPSG, PSG or the like. The thin film 402 has a thickness ranging from about 0.1 to about 2 .mu.m. An organic resist 403 is applied on the thin film 402 on the substrate 401. As the organic resist 403, well known are AZ1350, PFPR, TSMR, PMMA, and so forth. The thickness of the organic resist 403 ranges from about 0.1 .mu.m to about 2 .mu.m. The plate constructed from the organic resist 403, the thin film 402, and the substrate 401 is irradiated with an energy ray 405 such as an ultraviolet ray and an electron beam as shown in FIG. 4B. By selective exposure of the resist to the energy ray, a resist pattern 404 is formed. The ultraviolet ray or the electron beam is projected in a breadth of L.sub.1. The region L.sub.1 of the resist irradiated with the light or the electron beam is sensitized, and only the irradiated region of the organic resist is removed by immersion of the resist in a developing solution as shown in FIG. 4C, thereby forming a developed resist, i.e., a resist pattern 406. The above description was made in regard to the case of a positive type resist. In the case of a negative type resist, the organic resist at the irradiated region remains after the development. In the subsequent etching step as shown in FIG. 4D, the organic resist 408 serves as a mask, and the thin film at the portion uncovered with the resist 408 is etched off to form a pattern of the thin film 407.
In the conventional process as shown in FIGS. 4A to 4D, the breadth L.sub.2 at the development step in FIG. 4C and the breadth L.sub.3 at the etching step in FIG. 4D change slightly from the irradiation breadth L.sub.1 of the ultraviolet ray or the electron beam 405. Therefore, it is extremely difficult to achieve a sufficiently small final breadth L.sub.3, for example, of 0.1 .mu.m or less. In other words, because of the finite thickness of the photosensitive resist 403, the working dimension L.sub.2 reflected by sensitivity and developing properties of the organic resist becomes naturally different from L.sub.1, and L.sub.3 becomes different from L.sub.2 since not only the thin film but also the organic resist 408 is etched in the etching step.
As described above, in a conventional lithography, which is widely employed in an ultra LSI process, the finite thickness of the organic resist makes it extremely difficult to achieve the working dimension L.sub.3 of 0.1 .mu.m or less.